In the eye, the final stage of image processing in the retina is performed by retinal ganglion cells. The axons of the ganglion cells project out of the eye to form the optic nerve. Glaucoma, a disease which produces irreversible blindness if not treated early enough, destroys the ganglion cells. Until recently, the typical first sign of the presence of glaucoma has been the loss of a portion of the peripheral visual field, which is referred to as a "scotoma". Unfortunately, by the time a scotoma is detected, the disease has reached a stage where treatment can at best prevent further irreversible blindness.
One approach that has been used to screen subjects for glaucoma at an earlier stage than when a scotoma has developed is to test the intraocular tension of a patient. A symptom of glaucoma is the increase of intraocular tension. Tests of intraocular tension, however, usually involve the use of drugs, are time consuming, and are unpleasant for the subject. Moreover, some glaucoma sufferers do not exhibit intraocular tensions above about 21 mm of mercury (such subjects have what is called "normal-tension glaucoma"). Thus, testing the intraocular tension of a subject is not a reliable method for the early detection of glaucoma.
Another proposal for the early detection of glaucoma has involved the assessment of colour vision defects. Simple tests of colour vision defects, however, have shown a lack of correlation between the defects noted and the presence of optic disc cupping. More complex tests of colour vision defects involving anomaloscopy are too difficult for clinic use. Moreover, those tests cannot differentiate between colour vision defects caused by glaucoma and colour deficits resulting from amblyopia and optic neuritis. In addition, it has been reported that up to 25 per cent of subjects who have a glaucomatous scotoma exhibit no colour deficit. Thus assessment of colour deficits in a person's vision is not a reliable method of detecting glaucoma in its early stages, even if it should become practical to perform detailed colour vision tests clinically.
A conventional method for characterising damage by glaucoma involves perimetry. In a typical perimetric investigation, a series of small luminous dots are projected onto a screen which is placed in front of a subject. The subject's vision is assessed on the basis of whether the subject testifies to seeing each dot as it is presented. This procedure, being a serial search, is very time consuming and is prone to errors arising from a subject's fallibility. Moreover, this perimetric technique cannot be used selectively for assessing glaucomatous damage, for it provides only an indication of localised blind spots in the visual field of a subject, which may arise from a variety of retinal and central nervous system disorders which affect vision. In particular, perimetric investigations take no account of the spatial scale of the visual system, for they typically use dots of the same size in all parts of the visual field, and in no way do they cater to any visual subsystem which may be damaged by glaucoma.
A psycho-physical test for the onset of glaucoma which was developed by the present inventor, and which has been found to be an effective method of detecting the presence of glaucomatous damage at an early stage, is described in the specification of Australian patent No 611,585 and in the specification of the corresponding U.S. Pat. No. 5,065,767. That test involves the presentation of a sinusoidal grating pattern to a subject while the contrast of the pattern is modulated at a frequency in the range from about 10 Hz to about 50 Hz, so that the subject observes a frequency-doubled pattern of the grating. The contrast of the pattern is then reduced until a threshold value is reached, at which value the frequency-doubled pattern is no longer observed by the subject. This threshold value is then compared with the threshold value for persons having normal, healthy vision. A higher than normal threshold value indicates that the subject may be suffering from glaucoma. Persons with well-developed glaucoma have threshold values that are approximately twice the threshold value of a person zenith healthy vision. The pattern is conveniently established on the screen of a cathode ray tube, controlled by a programmed microprocessor.
The test of Australian patent No 611,585 and U.S. Pat. No. 5,065,767, although an effective, non-invasive, quick, and easy to perform test, reveals an average loss of ganglion cells on the retina of a subject due to glaucomatous damage. The loss of ganglion cells in one region of the retina may be masked if the threshold contrast value for the disappearance of the frequency-doubled illusion is only slightly below the average value for persons having healthy vision.